The present invention relates to a liquid crystal display, and more particularly to a liquid crystal display comprising an OCB-mode (Optically Self-Compensated Birefringence mode) liquid crystal display panel.
In recent years, with advance in multimedia technologies, a great deal of image information has been distributed. As a means for displaying such image information, liquid crystal displays have rapidly spread. This is because liquid crystal displays with high contrast and wide viewing angle have been developed and put to practical use with development of liquid crystal technologies. At present, the liquid crystal displays are equal to CRT (Cathode Ray Tube) displays in display performance.
However, current liquid crystal displays are not suitable for use in display of moving images because of a low response speed of liquid crystal. While it is required that the liquid crystal respond within one frame period (16.7 msec) in a current NTSC (National Television Standard Committee) system, the current liquid crystal displays require more than 100 msec to respond between gray scales in multiple gray scale display, thereby causing a displayed moving image to be blurred. In particular, since the response between gray scales in a region where a drive voltage is low is extremely slow, a satisfactory moving image display is not attained.
Accordingly, many attempts have been conventionally made to provide high-speed responsive liquid crystal displays. While various liquid crystal display methods of high-speed response have been summarized by Wu et al. (C. S. Wu and S. T. Wu, SPIE, 1665, 250 (1992)), methods capable of achieving a response characteristic necessary for displaying the moving image are limited.
Currently, liquid crystal displays comprising an OCB-mode liquid crystal display panel, a ferroelectric liquid crystal display panel, or an anti-ferroelectric liquid crystal display panel are believed to be promising as liquid crystal displays having high-speed responsiveness suitable for display of the moving image.
Among these liquid crystal display panels, the ferroelectric liquid crystal display panel and the anti-ferroelectric liquid crystal display panel having a layered structure suffer from many problems associated with their practical uses such as: low shock resistance, limited range of available temperatures, and high temperature dependency of property. In view of these, attention has been focused on the OCB-mode liquid crystal display panels using nematic liquid crystal as liquid crystal displays suitable for display of the moving image.
The high-speed responsiveness of the OCB-mode liquid crystal displays was demonstrated by J. P. Bos in 1983. Since it was thereafter demonstrated that the provision of retardation films brought about displays with wide viewing angle and high-speed responsiveness, the OCB-mode liquid crystal display panels have been studied and developed.
FIG. 36 is a cross-sectional view schematically showing a constitution of the conventional OCB-mode liquid crystal display panel. Referring to FIG. 36, the OCB-mode liquid crystal display panel comprises a first glass substrate 81 provided with a transparent counter electrode 82 on a lower surface thereof and a second glass substrate 88 provided with a transparent pixel electrode 87 on an upper surface thereof. A first alignment layer 83 is formed on a lower surface of a counter electrode 82 and a second alignment layer 86 is formed on an upper surface of the pixel electrode 87. Liquid crystal molecules have been filled into a gap between these alignment layers 83, 86 to be formed into a liquid crystal layer 84. The alignment layers 83, 86 have been subjected to alignment treatment to align the liquid crystal molecules in parallel with one another and in the same direction. The thickness of the liquid crystal layer 84 is defined by a spacer 85.
A first polarizer 91 is provided on an upper surface of the first glass substrate 81 and a second polarizer 92 is provided on a lower surface of the second glass substrate 88. These polarizers 91, 92 are provided in cross nicole, that is, such that their optical axes are orthogonal to each other. A first retardation film 89 is provided between the first polarizer 91 and the first glass substrate 81 and a second retardation film 90 is provided between the second polarizer 92 and the second glass substrate 88. As the retardation films 89, 90, negative retardation films whose main axes are hybrid-arranged are used.
In the OCB-mode liquid crystal display panel so constituted, by application of a voltage, the liquid crystal is caused to transition from spray alignment 84a to bend alignment 84b, in which state, an image is displayed. Since the response speed of the liquid crystal of the OCB-mode liquid crystal display panel is significantly improved a compared to a TN-mode(Twisted nematic mode) liquid crystal display panel, the liquid crystal display panel suitable for moving image display is realized. In addition, the provision of the retardation films 89, 90 can achieve wide viewing angle.
As described above, the OCB-mode liquid crystal display panel displays an image when the liquid crystal is in the bend alignment state. Therefore, an initialization process for transitioning from initial spray alignment to bend alignment (hereinafter simply referred to as spray-bend alignment transition) is essential.
FIGS. 37A-37C are views for explaining the initialization process for performing the spray-bend transition in the conventional liquid crystal display, wherein FIG. 37A is a graph showing change in the rate of the spray-bend transition, and FIGS. 37B, 37C are graphs each showing a waveform of a voltage applied to the liquid crystal display panel in the initialization process.
In FIG. 37A, a longitudinal axis indicates the rate of transition from initial spray alignment to bend alignment in the liquid crystal layer included in the liquid crystal display panel. In FIGS. 37B, 37C, longitudinal axes respectively indicate potential difference between the source line and the counter electrode and potential difference between the gate line and the source line.
As shown in FIG. 37B, in the initialization process, a predetermined voltage is applied intermittently to the source line and the counter electrode so that the potential difference between the source line and the counter electrode becomes 10 V or more. Also, as shown in FIG. 37C, a predetermined voltage is applied to the gate line and the source line so that the potential difference between the gate line and the source line becomes 10 V or more over the whole initialization process. As a result, as shown in FIG. 37A, the rate of transition to the bend alignment is increased stepwise and the spray-bend transition is completed when the initialization process is terminated.
By the way, how the spray-bend transition takes place is observed and the observation result shows that a nucleus of the bend alignment is generated from a specific spot and grown. Hereinbelow, this nucleus is named xe2x80x9ctransition nucleusxe2x80x9d.
Publication of Examined Patent Application No. Hei. 10-20284 discloses a liquid crystal display panel in which a convex/concave portion made of a conductive material is formed at a predetermined position on the side of an array substrate for the purpose of generating the transition nucleus. In this constitution, since the electric field strength applied to a region of the liquid crystal layer on the convex/concave portion becomes larger than that around the region, the generation of the transition nucleus is facilitated. Consequently, the spray-bend transition smoothly takes place.
However, in the conventional liquid crystal display, the spray-bend transition sometimes takes place with low reliability because of insufficient strength of the electric field. In this case, the spray-aligned region is locally left and becomes a luminescent spot, which is observed as dot defect.
The present invention is directed to solving the above-described problem and an object thereof is to provide a liquid crystal display capable of reliably performing spray-bend transition.
To solve the above-described problem there is provided a liquid crystal display comprising: a pair of opposed substrates; a liquid crystal layer disposed between the pair of substrates, the liquid crystal layer having a display alignment state and a non-display alignment state which differ from each other and being subjected to an initialization process so as to be changed from the non-display alignment state to the display alignment state, before an image is displayed; a first electrode provided on one of the pair of substrates; a second electrode provided so as to overlap with the first electrode with an insulator interposed there between and disposed between the first electrode and the liquid crystal layer, the second electrode having a lack portion in a region overlapping with the first electrode; and drive means for generating potential difference between the first electrode and the second electrode to thereby perform the initialization process.
In this constitution, when the potential difference is generated between the first electrode and the second electrode, the electric field strength around the lack portion included in the second electrode is larger than the electric field strength in the other region. As a result, the liquid crystal molecules around the lack portion become the transition nucleus and transition of the alignment state of the liquid crystal layer reliably takes place.
In the liquid crystal display, one of the pair of substrates may be an array substrate having a plurality of pixel electrodes provided in matrix; a plurality of gate lines and source lines arranged so as to cross each other; a plurality of switching devices provided as corresponding to the respective pixel electrodes, for switching between a conductive state and a non-conductive state between the pixel electrodes and the source lines in accordance with a drive signal supplied through the gate lines, and the other of the pair of substrates may be an opposing substrate having a counter electrode opposed to the array substrate.
The liquid crystal display may further comprise storage capacitor electrodes overlapping with the pixel electrodes, and the first electrode may be the storage capacitor electrode and the second electrode may be the pixel electrode.
In the liquid crystal display, the first electrode may be the gate line and the second electrode may be the pixel electrode.
The liquid crystal display, may further comprise storage capacitor electrodes overlapping with the pixel electrodes, and the first electrode may be the storage capacitor electrode and the second electrode may be the source line.
In the liquid crystal display, the first electrode may be the gate line and the second electrode may be the source line.
In the liquid crystal display, the first electrode may be the pixel electrode and the second electrode may be the gate line.
The liquid crystal display, may further comprise storage capacitor electrodes overlapping with the pixel electrodes, and the first electrode may be the pixel electrode and the second electrode may be the storage capacitor electrode.
In the liquid crystal display, the first electrode may be the source line and the second electrode may be the gate line.
The liquid crystal display, may further comprise storage capacitor electrodes overlapping with the pixel electrodes, and the first electrode may be the source line and the second electrode may be the storage capacitor electrode.
The liquid crystal display, may further comprise: a third electrode and a fourth electrode provided on one of the pair of substrates on which the first and second electrodes are not provided, so as to overlap each other with an insulator interposed therebetween, the third electrode may be disposed between the fourth electrode and the liquid crystal layer and has a lack portion in a region overlapping with the fourth electrode, and the drive means may be adapted to generate the potential difference between the third electrode and the fourth electrode to perform the initialization process.
In this constitution, when the potential difference is generated between the third electrode and the fourth electrode to perform transition of the alignment state of the liquid crystal layer, the electric field strength around the lack portion included in the third electrode is larger than the electric field strength in the other region. As a result, the liquid crystal molecules around the lack portion of the third electrode as well as the liquid crystal molecules around the lack portion of the second electrode, become transition nucleuses. By thus generating the transition nucleuses on the sides of both substrates, the transition of the alignment state of the liquid crystal layer can take place more reliably.
In the liquid crystal display, the lack portion may be an aperture provided in the second electrode.
In this case, the aperture may include a plurality of straight-line portions extending toward a position at which these portions cross each other. Also, the aperture may be V-shaped, W-shaped, or X-shaped. Further, the aperture may be polygon-shaped.
In the liquid crystal display, the lack portion may be shaped to enable application of two-direction electric fields to the liquid crystal layer. In this constitution, two types of, i.e., clockwise and counterclockwise twist-aligned regions may be formed. Since elastic strain energy is increased at a spot where these twist-aligned regions are in contact with each other, the transition of the alignment state of the liquid crystal layer smoothly takes place.
In the liquid crystal display, the second electrode has an aperture including a portion which is 4 xcexcm wide or less. In this constitution, the electric field strength around the aperture included in the first electrode can be made larger.
In the liquid crystal display, the lack portion may be a cutout portion provided in the second electrode. In this constitution, the liquid crystal molecules around the cutout portion become the transition nucleus and the transition of the alignment state of the liquid crystal layer can take place reliably.
According to the present invention, there is also provided a liquid crystal display comprising: a pair of opposed substrates; a liquid crystal layer disposed between the pair of substrates, the liquid crystal layer having a display alignment state and a non-display alignment state which differ from each other and being subjected to an initialization process so as to be changed from the non-display alignment state to the display alignment state before an image is displayed; a first electrode and a second electrode formed on one of the pair of substrates so as to overlap each other with an insulator interposed therebetween; drive means for generating potential difference between the first electrode and the second electrode to perform the initialization process; and convex portions respectively formed at opposed positions in the pair of the substrates such that the convex portions are protruded in the thickness direction of the liquid crystal layer.
In the constitution, the cell gap in the region with the convex portion is smaller than the cell gap in the region without the convex portion. Thereby, when the potential difference is generated between the first electrode and the second electrode to perform transition of the alignment state of the liquid crystal layer, the electric field strength can be locally increased around the cell gap in the region with the convex portion. As a result, the liquid crystal molecules around the cell gap become the transition nucleus and the transition of the alignment state of the liquid crystal layer can reliably take place.
According to the present invention, there is still further provided a liquid crystal display having: a pair of opposed substrates; and a liquid crystal layer disposed between the pair of substrates, the liquid crystal layer having a display alignment state and a non-display alignment state which differ from each other and being subjected to an initialization process so as to be changed from the non-display alignment state to the display alignment state before an image is displayed; comprising: a first electrode provided on one of the pair of substrates; a second electrode placed between the first electrode and the liquid crystal layer; and drive means for generating potential difference between the first electrode and the second electrode to thereby perform the initialization process, and opposed end portions of two adjacent second electrodes overlap with the first electrode with an insulator interposed therebetween.
In the constitution, when the potential difference is generated between the first electrode and the second electrode to perform transition of the alignment state of the liquid crystal layer, the electric field strength is locally increased between the opposed end portions of the adjacent second electrodes. As a result, the liquid crystal molecules around the region between the opposed end portions become transition nucleuses and the transition of the alignment state of the liquid crystal molecules can reliably take place.
In the liquid crystal display, one of the opposed end portions may have a protrusion in a region overlapping with the first electrode and the other end portion may have a recess corresponding to the protrusion in the region overlapping with the first electrode. In this constitution, the liquid crystal molecules around the region between the protrusion and the corresponding recess become transition nucleus and the transition of the alignment state of the liquid crystal layer can reliably take place.
In the liquid crystal display, distance between the protrusion and the recess may be 4 xcexcm-8 xcexcm. Thereby, without shorting between the first electrodes, the electric field strength between the protrusion and the corresponding recess can be increased.
In the liquid crystal display, the protrusion maybe saw-tooth shaped.
In the liquid crystal display, one of the pair of substrates may be an array substrate having a plurality of pixel electrodes provided in matrix; a plurality of gate lines and source lines arranged so as to cross each other; a plurality of switching devices provided as corresponding to the respective pixel electrodes, for switching between a conductive state and a non-conductive state between the pixel electrodes and the source lines in accordance with a drive signal supplied through the gate lines, and the other of the pair of substrates may be an opposing substrate having a counter electrode opposed to the array substrate.
The liquid crystal display, may further comprise storage capacitor electrodes overlapping with the pixel electrodes, and the first electrode may be the storage capacitor electrode and the second electrode may be the pixel electrode.
In the liquid crystal display, the first electrode may be the gate line and the second electrode may be the pixel electrode.
In the liquid crystal display, the insulator may be a color filter or a flattening layer. In this constitution, the color filter or the flattening layer can be used as the insulator between the first electrode and the second electrode.
In the liquid crystal display, an intermediate portion may be formed between a main portion of the second electrode and the end portion of the second electrode so as to have a width smaller than a width of the main portion and a width of the end portion.
In this constitution, by adjusting the width and length of the intermediate portion, the storage capacitance generated between the opposed end portions of the adjacent pixel regions and the storage capacitance generated by the other elements can be well-balanced.
In the liquid crystal display, the first electrode may be comprised of a conductive mask and the second electrode may be the counter electrode.
In the liquid crystal display, the potential difference is preferably 15 Vxe2x88x9232 V.
In the liquid crystal display, voltages of different polarities may be respectively applied to adjacent pixel electrodes. Thus, by applying the voltage by so-called dot inverting method, two-direction transversal electric fields can be generated. As a result, two types of, i.e., clockwise or counterclockwise twist-aligned regions can be formed. Since the elastic strain energy is increased at the spot where these twist-aligned regions are in contact with each other, the transition of the alignment state of the liquid crystal layer can take place more smoothly.
In the liquid crystal display, the non-display alignment state may be spray alignment and the display alignment state may be bend alignment. Thereby, a liquid crystal display capable of reliably performing spray-bend transition is realized.
The liquid crystal display, may further comprise: an illuminating device having a light source for emitting red light, green light, and blue light; and illuminating device control means for controlling the illuminating device so as to emit the red light, the green light and the blue light by time division within one frame period. Thereby, a liquid crystal display that employs so-called field sequential color method and is capable of reliably performing transition of the alignment state of the liquid crystal layer can be realized.
According to the present invention, there is still further provided a liquid crystal display comprising: a pair of opposed substrates; a liquid crystal layer disposed between the pair of substrates, the liquid crystal layer having a display alignment state and a non-display alignment state which differ from each other and being subjected to an initialization process so as to be changed from the non-display alignment state to the display alignment state before an image is displayed, and one of the pair of substrates may be an array substrate having a plurality of pixel electrodes provided in matrix; a plurality of gate lines and source lines arranged so as to cross each other; a plurality of switching devices provided as corresponding to the respective pixel electrodes, for switching between a conductive state and a non-conductive state between the pixel electrodes and the source lines in accordance with a drive signal supplied through the source lines, and the other of the pair of substrates may be an opposing substrate having a counter electrode opposed to the array substrate, and a source electrode constituting the switching device may extend from the source line in parallel with the gate line so as to overlap with the gate line and may be interposed between the gate line and the liquid crystal layer, and a drive signal for causing conduction between the pixel electrode and the source lines may be supplied to the gate lines to set the source electrode and the pixel electrodes at equipotential and potential difference is generated between the source line and the gate line to thereby perform the initialization process.
In the liquid crystal display, potential difference may be generated between the counter electrode and the pixel electrode.
In the liquid crystal display, the source electrode may have a bent portion.
This object, as well as other objects, features and advantages of the invention will become more apparent to those skilled in the art from the following description taken with reference to the accompanying drawings.